Organic light emitting diodes (OLEDs) are particularly useful for lighting because they can relatively easily and cheaply be fabricated to cover a large area on a variety of substrates. They are also bright and may be coloured (e.g. red, green and blue) or white as desired. In this specification references to OLEDs include organometallic LEDs, and OLEDs fabricated using either polymers or small molecules. Examples of polymer-based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so-called small molecule based OLEDs are described in U.S. Pat. No. 4,539,507.
To aid in understanding embodiments of the invention it is helpful to describe an example structure of an OLED device. It is worth noting that when operated under reverse polarity the OLED can function as an OPV. Thus referring to FIG. 1a, this shows a vertical cross-section through a portion of an OLED device 10 (e.g., display backlight OLED, OLED lighting tile or OLED lighting panel) comprising a glass substrate 12 on which metal, for example copper tracks 14, are deposited to provide a first electrode connection, in the illustrated example an anode connection. A hole injection layer 16 is deposited over the anode electrode tracking, for example a conductive transparent polymer such as PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This is followed by a light emitting polymer (LEP) stack 18, for example comprising a PPV (poly(p-phenylenevinylene)-based material. The hole injection layer helps to match the hole energy levels of the LEP stack to the anode metal. This is followed by a cathode stack 20, for example comprising a low work function metal such as calcium or barium (for the LEP stack and cathode electron energy level matching) or an electron injection layer such as lithium fluoride, over which is deposited a reflective back electrode, for example of aluminium or silver.
The example of FIG. 1a is a “bottom emitter” device in which light is emitted through the transparent substrate, e.g. glass or plastic. However a “top emitter” device may also be fabricated in which an upper electrode of the device is substantially transparent, for example fabricated from indium tin oxide (ITO) or a thin layer of cathode metal (say less than 100 nm thickness). Referring now to FIG. 1b this shows a view of the OLED device 10 of FIG. 1a looking towards the LEP stack 18 through the substrate 12, that is looking into the light-emitting face of the device through the “bottom” of the device. This view shows that the anode electrode tracks 14 are, in this example, configured as a hexagonal grid or mesh, in order to avoid obscuring too much light emitted from the LEP stack 18. The (anode) electrode tracks 14 are connected to a solid metal busbar 30 which runs substantially all the way around the perimeter of the device, optionally with one or more openings 32, which may be bridged by an electrical conductor to facilitate a connection to the cathode layer of the device.
FIG. 1c shows a Lighting Panel 100 comprising a plurality of OLEDs 10 having a structure as shown in FIGS. 1a and/or 1b. 
Metal tracking lines such as anode tracks 14 are provided in OLEDs to increase the conductivity of an electrode in the device and thus enable current distribution over a wide area. However, deposition of active OLED layers on top of a non-planar surface may result in thickness and/or contour variations, i.e., non-planar surface regions, of the layers. Such variations may for example result in luminance non-uniformities, device instabilities and/or electrical shorts in the device. Edges of the metal tracks may cause such thickness and/or contour variations. Thus, metal tracking in Lighting Panels comprising OLEDs is preferably planarised prior to processing of the light-emitting and associated (e.g., charge injection) layers.
The field of substrate planarisation however continues to provide a need for a planarisation technique that provides an improvement in relation to one or more of, interalia, surface planarity, process complexity, processing time, process/device cost, light outcoupling through a planarised surface, electrical conduction from tracking to a layer (e.g., hole injection or light emissive layer) associated with light emission, contamination of an active area of a lighting device, corrosion of metal tracks, OLED cavity tuning, etc.
For use in understanding the present invention, the following disclosures are referred to:                Proc. SPIE, Vol. 7415, 74150T (2009), Harkema et al;        Journal of Applied Physics 101 (2007) 064513, Zhao et al;        U.S. Pat. No. 5,010,027, Possin et al, published 1991 Apr. 23;        U.S. Pat. No. 5,597,747, Chen, published 1997 Jan. 28;        Presentation from Comedd-Opening on Oct. 30, 2008, “Organic Lighting and Organic Solar Cells”, Prof. Dr. Karl Leo, Fraunhofer IPMS, available from http://www.ipms.fraunhofer.de/common/comedd/presentation/leo.pdf;        Osram Datasheet “ORBEOS™ for OLED Lighting”, dated 2009 Nov. 18, available at least from May 18, 2010, available from http://www.osram-os.com/osram_os/EN/Products/Product_Promotions/OLED_Lighting/Technical_Information/index.html; and        Proc. SPIE, Vol. 6192, 61921Z-1 (2006), Fehse et al.        